Cancer is the second most common cause of death in the U.S. and accounts for one of every eight deaths globally (American Cancer Society, Cancer Facts and Figures, 2014). The American Cancer Society expects that in 2014 at least 1,665,540 new cancer cases will be diagnosed in the US and 585,720 Americans are expected to die of cancer, almost 1,600 people per day. Currently available paradigms for treating solid tumors may include systemic treatment such as chemotherapy, hormonal therapy, use of targeted agents and biological agents, either as single agents or in combination. These treatments can be delivered in combination with localized treatments such as surgery or radiotherapy. These anti-cancer paradigms can be use in the curative setting as adjuvant or neo-adjuvant treatments or in the metastatic setting as palliative case for prolonged survival and to help manage symptoms and side-effects. In hematological cancers, stem cell transplants may also be an option in certain cancers as well as chemotherapy and/or radiation. Although medical advances have improved cancer survival rates, there remains a continuing need for new and more effective treatments.
Ubiquitin is a small 76-amino acid protein that is the founding member of a family of posttranslational modifiers known as the ubiquitin-like proteins (Ubls). Ubls play key roles in controlling many biological processes including cell division, cell signaling and the immune response. There are 8 known human Ubl activating enzymes (known as E1s) (Schulman, B. A., and J. W. Harper, 2009, Ubiquitin-like protein activation by E1 enzymes: the apex for downstream signalling pathways, Nat Rev Mol Cell Biol 10:319-331). Ubiquitin and other Ubls are activated by a specific E1 enzyme which catalyzes the formation of an acyl-adenylate intermediate with the C-terminal glycine of the Ubl. The activated Ubl molecule is then transferred to the catalytic cysteine residue within the E1 enzyme through formation of a thioester bond intermediate. The E1-Ubl intermediate and an E2 enzyme interact, resulting in a thioester exchange wherein the Ubl is transferred from the E1 to active site cysteine on the E2. The Ubl is then conjugated, i.e. transferred, to the target protein, either directly or in conjunction with an E3 ligase enzyme, through isopeptide bond formation with the amino group of a lysine side chain in the target protein. Eukaryotic cells possess ˜35 ubiquitin E2 enzymes and >500 ubiquitin E3 enzymes. The E3 enzymes are the specificity factors of the ubiquitin pathway which mediate the selective targeting of specific cellular substrate proteins (Deshaies, R. J., and C. A. Joazeiro, 2009, RING domain E3 ubiquitin ligases, Annu Rev Biochem 78:399-434; Lipkowitz, S., and A. M. Weissman, 2011, RINGs of good and evil: RING finger ubiquitin ligases at the crossroads of tumour suppression and oncogenesis, Nat Rev Cancer 11:629-643; Rotin, D., and S. Kumar, 2009, Physiological functions of the HECT family of ubiquitin ligases, Nat Rev Mol Cell Biol 10:398-409).
Two E1 enzymes have been identified for ubiquitin, UAE (ubiquitin-activating enzyme) and UBA6 (Jin, J., et al., 2007, Dual E1 activation systems for ubiquitin differentially regulate E2 enzyme charging, Nature 447:1135-1138). UAE is the E1 responsible for the majority (>99%) of ubiquitin flux within the cell. UAE is capable of charging each of the approximately ˜35 E2 enzymes with the exception of Use1, which is the only E2 known to exclusively work with UBA6 (Jin et al., 2007). Inhibition of UAE is sufficient to dramatically impair the great majority of ubiquitin-dependent cellular processes (Ciechanover, A., et al., 1984, Ubiquitin dependence of selective protein degradation demonstrated in the mammalian cell cycle mutant ts85, Cell 37:57-66; Finley, D., A. et al., 1984, Thermolability of ubiquitin-activating enzyme from the mammalian cell cycle mutant ts85, Cell 37:43-55).
The cellular signals generated by ubiquitin are diverse. Ubiquitin can be attached to substrates as a single entity or as polyubiquitin polymers generated through isopeptide linkages between the C-terminus of one ubiquitin and one of the many lysines on a second ubiquitin. These varied modifications are translated into a variety of cellular signals. For example, conjugation of a lysine 48-linked polyubiquitin chain to a substrate protein is predominantly associated with targeting the protein for removal by the 26S proteasome. A single ubiquitin modification, or monoubiquination, typically affects protein localization and/or function. For example, monoubiquitination modulates the following: 1) the function of Histones 2a and 2b (Chandrasekharan, M. B., et al., 2010, Histone H2B ubiquitination and beyond: Regulation of nucleosome stability, chromatin dynamics and the trans-histone H3 methylation, Epigenetics 5:460-468), 2) controls the nucleo-cytoplasmic shuttling of PTEN (Trotman, L. C., et al., 2007, 3) ubiquitination regulates PTEN nuclear import and tumor suppression, Cell 128:141-156), 4) drives localization of the FANCD2 protein to sites of DNA damage (Gregory, R. C., et al., 2003, Regulation of the Fanconi anemia pathway by monoubiquitination, Semin Cancer Biol 13:77-82) and 5) promotes the internalization and endosomal/lysosomal turnover of some cell surface receptors, like EGFR (Mosesson, Y., and Y. Yarden, 2006, Monoubiquitylation: a recurrent theme in membrane protein transport. Isr Med Assoc J 8:233-237). Other forms of polyubiquitination chains occur on lysine positions 11, 29 and 63, impacting various cellular roles including cell cycle, DNA repair and autophagy (Behrends, C., and J. W. Harper, 2011, Constructing and decoding unconventional ubiquitin chains, Nat Struct Mol Biol 18:520-528; Bennett, E. J., and J. W. Harper, 2008, DNA damage: ubiquitin marks the spot, Nat Struct Mol Biol 15:20-22; Komander, D., 2009, The emerging complexity of protein ubiquitination, Biochem Soc Trans 37:937-953).
UAE-initiated ubiquitin conjugation plays an important role in protein homeostasis, cell surface receptor trafficking, transcription factor turnover and cell cycle progression. Many of these processes are important for cancer cell survival and it is believed that tumor cells may have increased sensitivity to UAE inhibition as a result of their rapid growth rate, increased metabolic demands and oncogene fueled protein stress. Preclinical studies with PYZD-4409, a UAE inhibitor, demonstrated this compound induced cell death in both leukemia and myeloma cell lines and induced anti-tumor activity in a mouse acute myeloid leukemia (AML model). (Xu, W. G., et al., 2010, The ubiquitin-activating enzyme E1 as a therapeutic target for the treatment of leukemia and multiple myeloma, Blood, 115:2251-59). Thus, UAE represents a protein homeostasis target opportunity for the treatment of cancer.
New combinations of therapeutic agents that provide a beneficial effect in the treatment of cancers are desirable in order to prolong patient's lives while maintaining a high quality of life. Furthermore, new combination therapies may provide an increased benefit as compared to each single agents used individually. This is especially true in the case where cancers may be resistant or refractory to currently available therapeutic regimens.